This invention relates to drive systems for threaded fasteners, tools for their manufacture and drivers for driving the fasteners.
Threaded fasteners commonly used in industrial applications typically are driven by power tools at high speeds and under high torque loads. Such conditions present difficult design considerations, particularly with respect to the drive systems and, more particularly, with threaded fasteners having a driver engageable recess in the fastener head. Ideally, such a drive system should be easily manufactured, both as to recess and head geometry as well as to associated tooling for forming the fastener head and drivers for engaging the recess. The strength of the head of the fastener should not be adversely affected by the recess. The driver should be easily insertable into and easily withdrawn from the recess. The driver and recess, when mated, should distribute the stress loads uniformly to avoid formation of highly localized regions of stress that might result in deformation of the recess, or driver, or both, leading to premature failure of the drive system. The drive system should resist cam-out of the driver from the recess when the fastener is driven. In many applications, it is very important that the fastener must be capable of withstanding several cycles, as in applications where the fasteners must be removed in order to repair or replace parts or to remove and replace access panels. The fastener drive system ideally should be capable of such repeated cycling, particularly in environments where the recess may become contaminated, painted, corroded or otherwise adversely affected in use. In such environments, it is essential that the drive system maintain driving engagement while applying torque in a removal direction. It may be necessary for the drive system to be capable of applying even higher levels of torque when removing the fastener, as may occur when the fastener is over-torqued during initial assembly, or where corrosion develops at the interface at the engaged threads, or if thermal cycling of the assembled components has placed increased stress on the fastener. Where one or more of these, and other, characteristics may present competing considerations, compromises of one in favor of another may be made.
A variety of recess and driver configurations are in common use, including a number of cross-recesses, such as those described in U.S. Pat. No. 24,878 (Smith et al.); U.S. Pat. No. 3,237,506 (Muenchinger) and U.S. Pat. No. 2,474,994 (Tomalis). Other fastener geometries include multi-lobe geometries of the type described in U.S. Pat. No. 3,763,725 (Reiland) and ribbed drive systems as described in U.S. Pat. No. 4,187,892 (Simmons). Also among the common recess configurations is the Allen system which is essentially a straight walled hexagonally shaped socket receptive to a similarly shaped driver. With the exception of the ribbed systems, the walls and faces of the driver and recess typically are designed to fit closely with each other in an effort to achieve face-to-face contact of the driving and driven surfaces. With cross-recess fasteners, such face-to-face engagement can occur only, if at all, when the driver is properly aligned and seated within the recess. As a practical matter, however, in order to enable the driver to be inserted into the recess, there necessarily must be some clearance between the two. The necessity for such clearance is even more critical with recesses having substantially vertical drive walls, as in the Reiland ""725 patent and Allen head systems. In all of these systems, the practical result of the necessity for such clearance is that substantial face-to-face, broad area contact between the driver and recess surfaces is seldom achieved, if at all. With most drive systems for threaded fasteners, the driver mates with the recess in the head in a manner that results in point or line contact rather than face-to-face broad area contact. The actual area of contact typically is substantially less than full face-to-face contact. Consequently, when torque is applied by the driver, the forces applied to the screw head tend to be concentrated in localized areas with resulting high localized stresses. Such localized high stress can plastically deform the recess, forming ramps or other deformations resulting in premature, unintended disengagement of the driver from the recess.
The foregoing difficulties have been recognized in the art. For example, U.S. Pat. No. 2,248,695 (Bradshaw) discloses a screw head and driver arrangement in which the driving and driven faces of the driver and fastener, respectively, are curved and located eccentrically with respect to the screw axis. In the Bradshaw fastener, any xe2x80x9csuitable curvaturexe2x80x9d such as circular or log spiral may be used as long as it is oriented to bind or lock together by frictional engagement. Notwithstanding Bradshaw""s teachings, later fastener drive systems, as those referred to above, do not appear to have adopted the Bradshaw teaching of relying on frictional engagement.
It is among the general objects of the invention to provide a drive system for a threaded fastener that achieves many of the desirable characteristics for threaded fastener drive systems with less compromise of competing characteristics than has been the case with the prior art.
The advantages of the invention may be achieved by configuring the driving and driven surfaces of the driver and fastener, respectively, to conform to a segment of a spiral and, particularly, in a spiral configuration that enables a substantial and generous clearance between the driver and the recess during insertion and removal of the driver but in which the fully seated driver is permitted to rotate to take up that clearance. The spiral configurations of the drive walls of the driver and the driver-engageable walls of the recess are such that when the spiral walls engage, they do so over a relatively broad area thereby applying and distributing the stress over that broad area. The spirally configured driving and driven walls are oriented to direct a major portion of the applied torque substantially normal to the fastener radius with little, if any, reliance on frictional, near-tangential engagement. The drive walls of the driver and the driver-engageable walls of the recess may be formed to be as close to vertical as is practical, although some draft angle of several degrees may be acceptable. The configuration of the drive walls enables high levels of torque to be delivered without inducing substantial axially directed cam-out forces tending to urge the driver axially out of the recess. By so minimizing the tendency for cam-out, the recess may be relatively shallow, thereby resulting in a stronger screw head than would be the case with a deep, conventional recess.
A recess embodying the invention preferably includes a central portion and a plurality of generally radial wings, at least two of which have at least one heightwise extending, spiral driver-engageable wall. Recesses may be provided in which the spiral drive surfaces engage only in an installation direction, or only in a removal direction, or in both installation and removal directions. Additionally, the spiral surfaces may be arranged to have different arc lengths and orientations so that the torque capacity can be higher in one direction than in the other. A screw thus can be made capable of transmitting higher torque loads in a removal direction than in the installation direction so that sufficient torque can always be applied to overcome the torque that was previously applied during installation.
In a further aspect of the invention, the spiral configuration of the engageable driver and recess faces is configured and located with respect to the longitudinal axis of the screw so that when the driver face meets its associated recess face, it will tend to do so uniformly along the common engaging area of the those faces. The spiral configuration of the driver face preferably is substantially identical to that of the recess face. Consequently, when the driver is rotated to bring its spiral face into engagement with the driven face of the recess, the full surface area of the driver will engage the corresponding full surface area of the recess. So engaged, continued torque will be applied from the driver to the recess with the major proportion of the force being applied in a direction substantially perpendicular to the radius of the fastener. The transverse cross-sectional contour of the driver is somewhat smaller than that of the recess so as to leave a generous clearance that enhances the ability for the driver to be inserted and removed from the recess. The spirally configured driver surfaces enable the clearance to be taken up during the initial portion of driver rotation so that when the driver and recess drive walls engage they will do so in broad face-to-face contact. In its most preferable form, the mating spiral faces of the driver and recess are configured and oriented so that their spiral faces will substantially parallel each other throughout the range of permitted free rotation of the driver within the recess. In a still further aspect of the invention, the spiral engageable walls of the driver and recess are confined to the region of the spiral at its innermost portion. The initial point of the spiral may be located at a defined radius from the longitudinal axis of rotation of the fastener and the length of the spiral extends to a point that is a multiple of that initial radius, up to about three or three and one-half times the initial radius.
In another aspect of the invention a punch is provided for forming a recess in the head of a fastener blank in which the punch includes a main body having an end contoured to form a portion of the fastener head and a nib adapted to form the spiral recess of the invention in a conventional two-blow header technique. The radial extending wings of the nib may include one or two spiral surfaces adapted to form complementary spiral surfaces when impacted against the head end of the fastener. In a further aspect of the invention, the drive system may be used with externally driven fasteners in which the driving tool defines the recess and the driven surfaces on the fastener are defined by peripheral surfaces on the fastener head.
It is among the objects of the invention to provide a drive system for a threaded fastener by which torque can be effectively transmitted while minimizing localized stresses in the screw head and the driver.
Another object of the invention is to provide a drive system for a threaded fastener in which the driver engageable surfaces in the fastener are formed with a spiral contour and are adapted to be engaged by identically spiral contoured driver elements.
A further object of the invention is to provide a drive system for a threaded fastener in which the driving and driven faces of the driver and fastener respectively provide a generous clearance between the driving surfaces when the driver is fully inserted in the recess while still enabling broad face-to-face driving contact of the driver and recess drive walls.
Another object of the invention is to provide novel tooling for a drive system for threaded fasteners.